It has been known in the art to prepare structural materials for thermal insulation purposes by reacting lime and siliceous material, such as diatomaceous earth, in an atmosphere of saturated steam. It has also been proposed to use a variety of silica sources, including the sodium alumino silicates of volcanic origin, such as expanded perlite. Processes for the manufacture of lime-silica thermal insulating materials are disclosed, for example, in U.S. Pat. No. 2,698,256 issued Dec. 28, 1954 to F. L. Shea, Jr. and H. L. Hsu; and U.S. Pat. No. 3,238,052 issued Mar. 1, 1966 to Mr. Burak and P. McAmespie.
The principal function of the thermal insulation material is to minimize heat losses by reducing the rate of heat flow. In addition to the insulation requirements, an article intended for thermal insulation uses must be heat resistant, non-flammable, and durable. Furthermore, structural materials suitable for thermal insulation purposes, such as pipe, board, and block insulation, must possess sufficiently high modulus of rupture and compressive strength to withstand transportation and fittings. To comply with environmental regulations, the insulation material decomposition products at elevated temperature must be neither corrosive nor noxious.
These requirements are met by the lime-silica insulation products made in accordance with the present invention. It has been particularly found that lime-silica bonded structural articles made in accordance with the method of the present invention have high modulus of rupture as compared to lime-silica insulating material presently available in the industry, whereby the handling performance of the insulating products is substantially improved.
In general, at the present time, natural diatomaceous earth and silica flour are the preferred silica sources for lime-silica bonded insulation products. The lime-silica reaction results in a hydraulic bond formed within an atmosphere of saturated steam. In most lime-silica products, the weight ratio of the lime to silica (hereafter referred to as "the C/S ratio") is maintained in the 0.75 to 1.00 range. The formation of the hydrated calcium silicates proceeds in the following sequence:
(1) Calcined lime and silica, whether diatomaceous earth or perlite, react to crystalize tobermorite CSH (II) with a C/S ratio of about 1.75; and PA1 (2) The CSH (II) modification reacts with more silica to form CSH (I), reducing the C/S ratio gradually to 0.80.
The morphology of tobermorite crystals generated in lime-silica products made from diatomaceous earth and perlite depends on the autoclaving conditions. With increasing autoclaving time, the needle-like tobermorite phase recrystalizes to platey tobermorite. This inversion results in reduced strength after 8 hours autoclaving time.
A preliminary reaction between the lime and silicous source takes place in the aqueous slurry mix especially at elevated temperatures. Depending on the C/S ratio of the batch, a gel forms in a reasonable time period, e.g., 1 hour. The gelation rate is generally accelerated by adding small quantities of alkali compounds to the formulation. To prevent stress corrosion on steel pipes, the available chlorine content of the lime-silica raw materials must be neutralized, and alkali compounds are most effective in neutralizing acidic impurities. Based on the larger surface area of diatomite, the gel rate is increased so much in the presents of alkali compounds, that the working time of the gel becomes too short resulting in low strength and brittle lime-silica products. Perlite exhibits a much smaller surface area than diatomite, and its gel time is less affected by the alkaline environment.
It can be seen, therefore, that the achievement of desired strength properties of lime-silica insulation products is a problem, and that use of perlite as the silica source is significantly advantageous. Where perlite can be used to form lime-silica structural articles suitable for thermal insulation purposes, and the strength properties of such structural products improved, a substantial commercial improvement can be realized. By the present invention, lime-silica bonded structural articles suitable for thermal insulation purposes having good strength properties and deriving from perlite as the silica source are produced.
In particular, it has been found that lime-silica bonded structural articles made in accordance with the present invention using perlite as the silica source have a modulus of rupture as much as 30% greater than that of lime-silica structural articles presently available made from diatomaceous silica sources.